1. Field of the Invention
The present invention relates generally to the manufacture of nuclear fuel rods to be incorporated into fuel assemblies for nuclear reactors and, more particularly, is concerned with an apparatus and method for applying an end plug to an end of a fuel rod tube such that the end plug is guided in an improved manner into engagement with the fuel rod tube end.
2. Description of the Prior Art
Fuel elements or rods for nuclear reactors commonly encase the fissile material in thin walled cladding or tubes which serve to support the nuclear fuel during the operation of the reactor. The nuclear fuel, which is usually in the form of cylindrical pellets of enriched uranium dioxide, must be isolated from the environment surrounding the tubes to prevent contact and chemical reactions between the fuel and other materials such as water in a pressurized water reactor. Thus, the nuclear fuel is ordinarily hermetically sealed in each thin walled tube by the use of a pair of opposite end closures or plugs.
It is critical that the end plugs themselves be impermeable and mechanically strong to contain the fissile products. It is equally critical that the mechanical connection of each end plug with an end of each tube be free of defects such as discontinuities, cracks and tube distortions which could eventually produce leaks.
The act of plugging the tube is a critical operation since a heavy press fit must be used to contain the force of a plenum spring concurrently as the end plug is fitted to the tube. Press fitting an end plug into a fuel rod tube conventionally involves guiding the plug from a position at the bottom of a stack of identical plugs arrayed in a storage and feeding magazine into a guide bushing and then into a tube end. As much as 1000 pounds pressing force is required to concurrently compress the plenum spring and seat the plug in the tube end.
Present methods of applying the end plugs use close fitting guides to align the plugs with the tube, such as illustrated in FIG. 2 of the first patent application cross-reference above. However, the guide bushing normally used must be sized somewhat larger than the maximum expected size of the end plug due to manufacturing tolerances. The necessary clearance between the plug and guide sometimes permits the plug to cock, or tip, as shown in solid line form in FIG. 2 of the first cross-referenced application, resulting in a non-square press fit which can cause shearing at the tube/plug interface, that is, a shaving action on the plug exterior as the plug is rammed home. Thereafter, when the connection is completed such as by welding the plug to the tube, faulty welds commonly result which produce leaking because of discontinuities in the weld joint or connection caused by this shaving action. Also, frequently the plug does not seat properly which increases the likelihood of leakage. Additionally, pieces of the plug are broken off occasionally as it is forced into the tube. The broken pieces not only damage the end plug but also cause foreign objects to be deposited in the fuel rod.
One set of possible solutions to the above-described problems is represented by the alternative types of conforming guides described and illustrated in the first patent application cross-referenced above which have the common characteristic of holding the plug yet conforming to changes in size due to tolerance limitations. Notwithstanding the potential benefits to be gained in the use of a conforming guide, a need still exists to explore and develop improved alternative techniques for applying an end plug to the tubular end of a fuel rod which will increase the resistance of fuel rods to leakage and failure.